One of the usual ways of controlling an oil producing plant from the surface has been that of conveying both the power and the signals to operate moving parts (valves, retainers, flow diverters, etc.) along one same pair of wires while trying as much as possible to avoid interference between power and signal lines due to unwanted harmonics.
This practice, however, has its drawbacks, for instance:
a) since the undersea oil producing rigs are being laid in ever deeper waters, power-carrying cables have to travel long distances and therefore resistance becomes greater, which means that relatively high voltages have to be employed (100 to 400 V) and low frequencies (50 to 60 Hz), which is not always desirable because high voltages lead to power leaks along such undersea cables and connectors; PA1 b) signals sent out are usually of low amplitude and high frequency (typical figures being a few mV up to 5 V and 1 to 10 kHz) in order to avoid the appearance of harmonics in the power lines which would affect signals transmitted and make them difficult to read, and which would otherwise call for powerful and efficient filters to separate power from signals, which filters are expensive, bulky and complex and therefore not always as reliable as might be wished. PA1 it diminishes the causes of likely interference without the use of filters; PA1 it operates with both power and signals being conveyed by the same conductor (regarded as a more suitable way in an improvement already referred to); PA1 square waves are employed, and; PA1 above all, power does not have to be separated from signals, there being therefore no need for power to be turned off when signals are being sent.
The conveying of power and signal currents along the same pair of wires but at greatly different frequencies is known as frequency multiplexing.
There is another system that has been worked out whereby power is conveyed in the shape of square waves at typical figures, for instance, of 36 V and 50 Hz and signals are conveyed, for instance, at 3 V and 1 kHz, just one pair of conductors being used for both power and signals. As mentioned, although the voltage applied is low compared with the usual method referred to before, and the square waves used to convey power being very efficient when resistance in the interconnecting cable is very high, this method suffers from the drawback that signal has to be sent when power is turned off, which is referred to as time multiplexing. And, since power must be off when signals are being sent then it must also be off when any answer comes in from the end operating point in reply to the starting point. This latter aspect has not yet been successfully overcome. Also, even though requirements are less it has not been possible to do away with filters.